Semiconductor light sources, such as light emitting diodes (LEDs), are expanding into the area of lighting. The importance of a power supply unit (PSU) is increasing in LED lighting in comparison to conventional lighting, where maintenance and repair costs for a lighting device are increased due to a short life of the PSU. Also, an additional spatial design is necessitated due to the presence of the PSU. To overcome these deficiencies, an alternating current (AC) driving LED is demanded to produce an LED light source without a driver, such as a conventional filament light source. Accordingly, a high driving voltage is necessary to achieve bidirectional operation and to increase power efficiency.
Since the AC driving LED basically has a high forward voltage, light is not generated and a current does not flow at a low AC portion. Optically, this is shown as repetition of ON and OFF states, that is, a flicker phenomenon. Electrically, a phase difference between a voltage and a current is caused, thereby reducing electrical quality of the LED. Related electrical characteristics include a low power factor correlation (PFC) and a high total harmonic distortion (THD). The low PFC represents an unsatisfactory power efficiency. The high THD refers to a possibility of malfunction owing to a jammer transmitted to peripheral devices.
A conventional AC driving LED may have a current and voltage waveform as shown in FIG. 1. Referring to FIG. 1, when a phase difference between a current 120 and a voltage 110 is great, low PFC characteristics are undesirably obtained. As the waveform of the current 120 expands from a sine wave to a square wave, THD characteristics are increased, causing interference of electromagnetic waves. Furthermore, flicker characteristics may be reduced since the waveform of the current 120 corresponds to light emission.
To overcome the aforementioned limits, a circuit having a current and voltage waveform as shown in FIG. 2 is necessitated because the THD characteristics and light quality are increased as a waveform of a current 220 is more similar to a waveform of a voltage 210, which is a sine wave. In addition, since the current 220 flows even at an input voltage, a phase difference between the voltage 210 and the current 220 may be reduced and, accordingly, the PFC characteristics may be increased.
A driver that uses an integrated circuit (IC) may be used to produce a circuit having the current and voltage waveform as shown in FIG. 2.
FIGS. 3 and 4 illustrate light emitting circuits applying conventional ICs. The light emitting circuits using conventional ICs 310 and 410 may selectively drive a light emitting diode (LED). Therefore, in the conventional light emitting circuits, an LED Vf1 is first turned on at a low voltage so that a current flows and, according to an increase in an input voltage, LEDs are turned on in order of Vf2 and Vf3. Thus, the current and voltage waveform as shown in FIG. 2 is achieved. However, when the ICs are used in the conventional examples of FIGS. 3 and 4, the cost is increased and the circuits become undesirably more complicated.
Thus, the need exists for further improvements in light emitting circuits without complicated designs and increased manufacturing and costs.